The present invention relates to a water-gas-shift-reaction catalyst used for converting and removing carbon monoxide (CO) in a gas which is rich in hydrogen through a water gas shift reaction, a method for removing carbon monoxide in a hydrogen gas using such catalyst, and a fuel cell generation system using such catalyst.
A reformed gas which is rich in hydrogen is produced by a steam reforming reaction of a hydrocarbon gas, liquid or solid, or a fuel such as methanol or the like with steam, and such reformed gas contains carbon monoxide as a by-product. The reformed gas is used for a supply of hydrogen as a fuel in a fuel cell electricity generation system.
It is contemplated that among fuel cells, a polymer electrolyte fuel cell (or solid polymer fuel cell) is used for a vehicle, a compact generator and a domestic co-generation system since it operates at a lower temperature, produces a higher power density, and is expected to be compact and light and to have a shortened operation period.
The polymer electrolyte fuel cell uses a perfluorosulfonic acid based polymer membrane as a proton conductive solid electrolyte and operates at a temperature between 50xc2x0 C. and 100xc2x0 C. However, since the polymer electrolyte fuel cell operates at such a lower temperature, it is likely to be poisoned by an impurity contained in the reformed gas which is rich in hydrogen. Particularly, platinum used for an electrode in a fuel cell unit is likely to be poisoned by CO, and generation performance of the unit is degraded when CO is contained in the reformed gas beyond a certain predetermined concentration.
Thus, a CO removal unit is provided downstream a reforming unit which produces the reformed gas rich in hydrogen from the fuel, and CO is selectively converted and removed through a water gas shift reaction (CO+H2Oxe2x86x92CO2+H2) so that a concentration of CO in the reformed gas is usually reduced to not greater than 1%. A catalyst for the water gas shift reaction is used for the CO removal as a CO conversion catalyst, and hitherto a Cuxe2x80x94Zn based catalyst has been conventionally used. It is noted that in the generation system using the polymer electrolyte fuel cell, a selective oxidation reaction unit is additionally provided downstream the CO removal unit so that the CO concentration in the hydrogen gas is further reduced for example to not larger then 50 ppm, and the hydrogen gas of which CO concentration is thus reduced is supplied to a fuel cell unit.
However, since an activity of the Cuxe2x80x94Zn based catalyst is low, a large amount of the catalyst has to be used in order that the CO concentration in the hydrogen gas is reduced to not larger than 1%. Further, its activity ages, and therefore the catalyst has be replaced with a fresh catalyst periodically. Therefore, it is difficult to apply the Cuxe2x80x94Zn based catalyst to a fuel cell generation system wherein start and stop are repeatedly carried out, and in particular to a compact fuel cell generation system.
The present invention has been made considering the problem as described above, and an object of the present invention is to provide a catalyst for a water gas shift reaction which can removes CO effectively in a hydrogen gas preferably within a broader temperature range and which can be used for the removal of CO contained in the hydrogen gas. Such catalyst can be used as a catalyst which accelerates a water gas shift reaction when CO is removed from a gas rich in hydrogen which is formed particularly in a fuel cell generation system.
Another object of the present invention is to provide a method for reducing carbon monoxide in a hydrogen gas in which method the hydrogen gas containing carbon monoxide contacts with the catalyst for the water gas shift reaction catalyst as described above.
A further object of the present invention is to provide a fuel cell generation system characterized in that a hydrogen gas which contains carbon monoxide contacts with the catalyst for the water gas shift reaction, and an obtained hydrogen gas of which carbon monoxide content is reduced is supplied to a fuel cell unit.
In the first aspect, the present invention provides a catalyst for a water gas shift reaction characterized in that at least platinum is supported as an active component on a metal oxide carrier.
In the catalyst according to the present invention, the metal oxide carrier is preferably at least one selected from the group consisting of zirconia, alumina, titania, silica, silica-magnesia, zeolite, magnesia, niobium oxide, zinc oxide and chromium oxide (it is to be noted that a metal herein includes also silicon). Among those oxides, zirconia is particularly preferable. Also, a carrier of zirconia, alumina, silica, silica-magnesia, zeolite, magnesia, niobium oxide, zinc oxide or chromium oxide coated with titania may be used as the metal oxide carrier.
As to the catalyst according to the present invention, an amount of the active component supported by the carrier is preferably between0.1% by weight and 10.0% by weight converted into a platinum metal expression based on a weight of the carrier (that is, based on a weight of the carrier itself which does not include the active component, which is also applicable hereinafter to the amount of the a supported active component).
According to one embodiment, rhenium is supported as another active component in addition to platinum in the catalyst of the present invention. In this case, an amount of supported rhenium (converted into a rhenium metal expression) is preferably between 0.1% by weight and 10.0% by weight based on a weight of the carrier.
In a further embodiment, at least one other metal selected from the group consisting of yttrium, calcium, chromium, samarium, cerium, tungsten, neodymium, praseodymium, magnesium, molybdenum and lanthanum is further supported as a further active component in the catalyst of the present invention in addition to platinum and rhenium optionally present as described above. In this case, an amount of the further supported active component(s) (converted into a metal expression) is in total preferably between 0.1% by weight and 10.0% by weight based on a weight of the carrier.
As to the catalyst according to the present invention, any suitable manner may support the active component on the carrier. For example, a salt of a metal which is suitable for the active component is dissolved in a suitable solvent such as water, and thus obtained solution is mixed with the carrier (for example in a powder form) to produce a mixture (for example a slurry) before the mixture is dried (preferably dried by heating), so that the carrier is obtained as the catalyst for the water gas shift reaction on which carrier the active component is supported.
The obtained catalyst for the water gas shift reaction is preferably subjected to firing thereafter. In the present invention, firing means a treatment wherein the carrier on which the active component is supported is kept at an elevated temperature, and makes it possible to produce a catalyst of which activity is higher. For example, the carrier is kept at a temperature between 400xc2x0 C. and 600xc2x0 C. (for example, 500xc2x0 C.) for a period between one hour and six hours (for example, two hours) in a suitable atmosphere (for example, in air or in an inert atmosphere).
In other embodiment, in place of or before firing, the catalyst for the water gas shift reaction which is produced by the above supporting is preferably subjected to a washing treatment. The washing treatment is carried out using water, preferably warm water and more preferably hot water (for example, boiling water). Concretely, the washing treatment is so carried out that the carrier which includes the active component thereon is dispersed in water which is agitated, and then the carrier is separated (for example, by filtration) followed by drying.
In the second aspect, the present invention provides a process of producing a catalyst for a water gas shift reaction which removes carbon monoxide in a hydrogen gas, which process comprises the steps of:
(1) producing a slurry by mixing a metal oxide carrier and a solution (preferably an aqueous solution) of a platinum salt, and
(2) drying thus obtained slurry so as to obtain a dried carrier. According to this process, the catalyst for the water gas shift reaction according to the present invention as described above is produced.
The process according to the present invention may further comprise the step of (3) firing the dried carrier thus obtained. In another embodiment, in place of or before carrying out the firing step, the process may further comprise the step of (4) washing the obtained dried carrier or the fired carrier and particularly washing it with hot water. It is noted that in the process according to the present invention, the platinum salt is converted to platinum as the active component and supported during drying the slurry drying (step (2)), and further during the firing (step (3)) and/or during the washing (step (4)).
The explanations as to the catalyst of the present invention as described above are applicable to the metal oxide carrier, the amount of the supported platinum, the firing, and the washing and so on in the process of producing the catalyst according to the present invention. In addition, rhenium in addition to platinum may be supported by the carrier, in which a solution of a rhenium salt is used in place of the solution of the platinum salt.
As to supporting rhenium, rhenium together with platinum may be supported by the carrier, in which an aqueous solution containing both the rhenium salt and the platinum salt together is prepared (or an aqueous solution containing the platinum salt and an aqueous solution containing the rhenium salt are mixed), and the steps described above are carried out using the obtained solution (or the mixture). Alternatively, the above steps (1) and (2) in series are repeated twice wherein one component is supported first and the other component is supported subsequently so that the carrier is obtained which supports platinum and rhenium.
In the process according to the present invention, in addition to platinum or in addition to platinum and rhenium as the additional active component, at least one other metal as described above may be supported by the carrier as a further additional active component. In this case, all of the metals may be supported together or each of the metals may be supported separately as in the case of rhenium as described above. It is noted that as to amounts of rhenium and the other metal to be supported, the above explanations as to the catalyst of the present invention are applicable.
Further, in the third aspect, the present invention provides a method for removing carbon monoxide from a hydrogen gas containing carbon monoxide characterized in that the hydrogen gas is contacted with a catalyst for a water gas shift reaction which comprises a metal oxide carrier supporting at least platinum. As the catalyst for the water gas shift reaction in this method, the catalyst according to the present invention which is described above can be used.
In this method, the hydrogen gas which contains carbon monoxide is contacted with the catalyst for the water gas shift reaction which is retained by a proper means or manner at a temperature between 200xc2x0 C. and 400xc2x0 C. and preferably a temperature between 220xc2x0 C. and 350xc2x0 C. in the presence of steam. The steam may be supplied in any suitable manner. The hydrogen gas which is used in this method is not particularly limited as far as it contains carbon monoxide. For example, the method according to the present invention is preferably applied to a reformed gas which contains hydrogen as a main component, and also contains carbon monoxide and steam so that a concentration of carbon monoxide contained in the gas is reduced.
Thus, in the fourth aspect, the present invention provides a fuel cell generation system which comprises a CO removing unit which contains a catalyst for a water gas shift reaction comprising a metal oxide carrier which supports at least platinum, which system is characterized in that the CO removing unit contacts a reformed gas as a hydrogen gas containing carbon monoxide produced by a steam reforming unit with the catalyst for the water gas shift reaction, whereby the hydrogen gas of which carbon monoxide concentration has been reduced is produced. The hydrogen gas of which carbon monoxide concentration has been reduced is supplied to a fuel cell unit. It is noted that when thus reduced carbon monoxide concentration in the hydrogen gas has to be further reduced before the hydrogen gas is supplied to the fuel cell unit (for example in the case of the polymer electrolyte fuel cell generation system), the hydrogen gas of which carbon monoxide concentration has been reduced through the CO removing unit is passed through a selective oxidation unit before it is supplied to the fuel cell unit.